Conversion from electrical energy into heat is traditionally achieved with a resistive heater element in series with a control element. When electrical current passes through the resistive heater element, heat is produced in proportion to the resistance and to the square of the current. In a known linear control system, a control element adjusts current flow through the resistive heater element to produce a desired amount of heat. The control system often operates in response to a signal from a temperature sensor that is placed in the vicinity of a heat load. A disadvantage of this linear approach is that a significant amount of heat is dissipated in a device controlling the current flow through the resistive heater element; and thus, efficiency of the system is low.
In another known system, a controller drives an output switching device such that, it is either fully on or off. This approaches an ideal switch with zero dissipation. Proportional control is obtained by controlling the relative on and off times of the switching device, that is, its duty cycle, rather than the current amplitude. If the duty cycle, that is, a complete on-off cycle, is made considerably faster, for example, at least an order of magnitude faster, than a thermal time constant of the system, this system performs as well as the linear system described above without the power dissipation or loss in the controller. As in the above linear system, the controller often senses temperature and adjusts the duty cycle and hence, the power dissipated by the resistor, so that the load achieves the desired temperature.
Both of the above systems are capable of producing very good static temperature control performance, and considerable prior art exists for achieving this performance. However, there are at least two inherent disadvantages to these systems. A first disadvantage relates to dynamic capability. In order to provide a fast thermal response, the resistive heater element must be sized so that it can produce considerably more heat than is needed to maintain the load at a desired temperature. This implies that, at 100 percent duty cycle operation, that is, fully on, the resistive heater element must be capable of raising the temperature of the load considerably above a desired temperature set point. Often, it is not economically feasible to provide a control system that regulates the voltage across the heater element; and therefore, the maximum power that is dissipated under a 100 percent duty cycle may vary considerably under variable voltage conditions. This is because the power that is dissipated in a fixed value resistor varies with the square of the applied voltage. Thus, a fifty percent increase in voltage more than doubles the power that must be dissipated. Depending upon the nature of the load, this often produces excessive temperatures, either for the resistive heater element or the load. The effect is that the thermal gain of such a system is proportional to the square of the voltage across the heating element.
A second disadvantage relates to operation of the heater under failure conditions. Most heaters fail to an open circuit, a short circuit, or a lowered resistance condition. An open circuit condition usually poses no concern because current flow through the heating element ceases, and the heater operation just stops. A short circuit condition in a heater that is properly protected by a high current fuse or circuit breaker is not a problem if the current to the heater element is interrupted promptly. The difficulty occurs when the heater partially shorts, producing a lower resistance than expected. A heater resistance value may occur that produces a current that is too low to trip the protective device, yet is high enough to produce excessive power. If the controller cannot act fast enough or if it has failed, then excessive temperatures may result.
Another failure mode of concern is where a system control failure causes the switching stage to remain on at a 100 percent duty cycle, which provides maximum current flow in the heater for an extended period. In this situation, the overcapacity of the heater may be enough to develop excessive temperatures without tripping a protective device.